Adjustable waveguide elements



April 10, 1956 R D1505 2,741,745

ADJUSTABLE WAVEGUIDE ELEMENTS Filed Aug. 14, 1952 MAM?- United StatesPatent ADJUSTABLE WAVEGUIDE ELEMENTS Richard A. Dibos, Abington, Pa.,assignor to Philco Corporation, Philadelphia, Pa., a corporation ofPennsyivania Application August 14, 1952, Serial No. 304,433

14 Claims. (Cl. 333-31) This invention relates to electrical circuitelements and more particularly to adjustable circuit elements for use inwaveguides.

Adjustable dielectric phase shifters for waveguides have been in use forseveral years. In one form they comprise a single slab of dielectricmaterial disposed Within the waveguide. This slab has a cross-sectionalarea small compared to the cross-sectional area of the waveguide, and alength equal to several wavelengths of the energy to be phase shifted.The slab is mounted parallel to the longitudinal axis of the waveguideand is movable normal to its broadest surface in the direction of thelonger transverse axis of the waveguide. The slab may be supported inits position parallel to the narrow walls by two or more rods extendingtransversely through the narrow walls and the dielectric slab in adirection perpendicular to the electric field within the waveguidethatis, parallel to the broad walls of the waveguide. The phase shiftproduced by such a dielectric slab is a function of the transverseposition of the slab. While the variation in phase with changes in thetransverse position of the slab is not susceptible of simplemathematical expression, it has ben shown experimentally and it is nowgenerally known that the magnitude of the phase shift and the rate ofchange of phase shift for a unit change of position of the slab are muchgreater for positions of the slab near the center of the transversecross-section of the waveguide than for positions adjacent either of thenarrow walls. This is to be expected since the intensity of the electricfield within a waveguide is much greater near the center of thetransverse cross-section than it is near the narrow walls.

This rapid change in total phase shift for positions near the center ofthe cross-section is highly disadvantageous in precision phase shifterssince small changes in position of the dielectric slab producedisproportionately large shifts in phase. In order to secure a moreslowly varying phase shift characteristic, it is of course possible toreduce the length and/or cross-section of the dielectric slab, but onlywithin limits of practicability. Prior art phase shifters have attemptedto overcome this disadvantage by providing means for micrometricallyadjusting the position of the dielectric slab. Even this expedient isnot entirely satisfactory if the phase shift to be produced is of such amagnitude that the slab must be positioned near the center of thewaveguide. In instances where the total phase shift introduced must beprecisely known this shortcoming of prior art phase shifters cannot becompletely overcome by employing two phase shifters in series, one togive a fine adjustment of the phase and one to give a coarse adjustmentof the phase, since there is no way of accurately determining the exactamount of phase shift introduced by the latter phase shifter. Thesedisadvantages are particularly noticeable in phase shifters designed toproduce a large total phase shift that is adjustable over a range smallcompared to the maximum phase shift.

Adjustable waveguide attenuators have been constructed along linessimilar to the dielectric phase shifter described above by substitutinga resistive strip for the dielectric slab. The attenuation produced bythe resistive strip is again a function of the transverse position ofthe strip Within the Waveguide and is greater with the strip positionednear the center of the waveguide cross-section than it is when the stripis positioned adjacent one of the narrow walls. The rapid change inattenuation for positions of the resistivestrip near the center of thecrosssection has presented a problem similar in many respects to theproblem discussed above in connection with the dielectric phase shifter.

At one time an attempt was made to improve the frequency responsecharacteristic of sliding vane waveguide attenuators by providing tworesistive strips that were driven simultaneously from opposite sides ofthe waveguide towards the center. However, it was found that anattenuator of this type was very difiicult to construct. In addition itwas found that this design doubled the already rapid rise of attenuationfor positions of the dielectric strips near the center of the waveguidecrosssection. For these and other reasons the double vane or stripattenuator fell into disuse.

Therefore, it is an object of the present invention to provide new andimproved adjustable circuit elements for use in waveguides.

Another object of the present invention is to provide a new and improveddielectric phase shifter for use in waveguides.

A further object of the invention is to provide a novel Waveguideattenuator that may be adjusted with great precision.

Another object is to provide a dielectric phase shifter having gradualvariation in phase shift per unit of displacement of the phase shiftingelement.

A further object of the invention is to provide a novel attenuatorhaving a highly desirable control characteristic.

These and other objects of the invention which will appear as thedescription of the invention proceeds are achieved by employing twodielectric slabs or resistive strips disposed on opposite sides of theshorter transverse axis of the waveguide and means for moving thedielectric slabs or resistive strips at the same or at selectivelydifferent rates in the direction of the longer transverse axis of thewaveguide. With the slabs arranged in this manner, movement of both ofthem in the same direction with respect to the walls of the waveguidecauses the slabs to move in opposite directions with respect to thecenter of the waveguide. Movement of the slabs in opposite directionswith respect to the center causes the phase shift produced by one slabto increase while the phase shift produced by the other slab decreases.The increase and decrease are caused to occur at different rates so thata net change in phase shift is produced by displacement of the slabs,this shift being much smaller per unit displacement of the slabs thanthe change produced by conventional phase shifters due to thediiferential effect noted above. Movement of the resistive strips inopposite directions with respect to the center results in a similardifferential change in attenuation and a correspondingly small totalchange in attenuation per unit shift of postition of the resistivestrips.

For a better understanding of the present invention together with otherand further objects thereof reference should now be made to thefollowing detailed description which is to be read in connection withthe accompanying drawings, in which:

Fig. 1 is an isometric view, partially broken away, of a preferredembodiment of the invention;

a eas 7 3 Fig. 2 is an end view of .the preferred embodiment shown inFig. 1;

Fig; 3 is a graph of phase shift and attenuation versus slab displacement for ai single slab phase shifter or attenuator; 1 Fig.4 is a graphof phase shift and attenuation versus displacement for the double slabphase shifters or atteiiuator of Figs. 1 and 5; and

V "Fig. 5 is an end view 'of a second embodiment of the presentinvention. 7

Because the dielectric phase shifter and the resistive stfip attenuatorof the present invention are so nearly alike in' constructionandhoperation the following description will be limitedto'the dielectricphase shifter. At the end of the descn'ption the differences between thephase shifter and the attenuator will be discussed.

As shown in Fig. 1, the preferred embodiment of the invention comprisestwo slabs 10 and 12 of suitable di- 7 electric material such aspolystyrene or polyglas disposed parallel to the narrow walls 14 and 16of the rectangular waveguide 18. The ends of dielectric slabs 19 and 12are preferably tapered for a distance equal to approximately a'halfwavelength as measured in waveguide 18 in order to prevent reflectiontherefrom. Slabs 10 and.

12 are supported by two transversely positioned rods 20 and 22 which aresecured to slabs 10 and 12 and which pass through holes in walls 14 and16 and are slidable therein. Rods'20 and 22 are so spaced that energyreflected by one of the rods is cancelled by energy reflected by theother rod. Rods 20 and 22 are rigidly joined at the ends by transversecross members 24 and 26 which'ca'use rods 20 and 22 to move together andthus maintain slabs 10 and 12 parallel to the narrow walls 14 and 16 ofthe waveguide. Motion of rods 20 and 22, and hence of slabs 10 and 12,is effected by means 'of a micrometer screw of conventionalformromprising an internally threaded hub portion 32 which is affixed tocross member 24, and an externally threaded spindle portion 30 having asleeve and thimble 34. The end of spindle 30 is biased against pad 31,secured to the narrow wall'14 of the waveguide, by compression springs28 and 29. Hub 32 and sleeve 34 may be providedwith the usual'indicia toindicate theprecise positi n of the slabs. 7

Fig.2 is an end view of the embodiment of Fig. 1 taken from the lefthand end as shown in Fig 1. Parts in Fig. 2 corresponding to like partsin Fig l have been given the same reference numerals. The thickness t ofdielectric slabs 10 and 12 will depend somewhat upon the phase shift tobe produced. In general, the longer and thicker the slabs are made, thegreater will be the resulting phase shift. However, if the dielectricslabs are made too thick or if the width D of the waveguide is made toolarge, more than one mode may be propagated in the waveguide. Ingeneral, the higher the di- 1 electric constant of the dielectric slab,the thinner the slab must be made if undesirable modes are to beeliminated. In a typical embodiment of the invention the dielectricslabs may have a length equal to 3 or 4 wavelengths as measured in thewaveguide 18 and a thickness z of the order of of a wavelengthat theoperating frequency. The clearance between the dielectric slabs and thebroader waveguide walls 36 and 38 will depend somewhat upon the power tobe transmitted through the Waveguide. in high power transmission systemslarger gaps may be required in order to prevent arcing between the walls36 and 38 and the dielectric slabs 10 and 12. In low power transmissionsystems the clearance need only be sutficient toprevent'undue frictionbetween the dielectric slabs and the broader walls 36 and 38 of thewaveguide 18. The spacing between slabs 10 and 12 is preferably equal toone-half of the width D of waveguide 18- However, the spac- .ing may bemade greater or less than this amount within reasonable limits.

Fig. 3'illustrates the variation in phase shift produced by moving thesingle slab of a conventional prior art phase shifter completely acrossthe broader dimension of a waveguide. The action of such a phase shiftermay be visualized by assuming that slab 12 is not present in the phaseshifter shown'in Fig. 2 and that dielectric slab 10is free to be movedfrom left to right throughout the entire width 1) of waveguide 1 8; Theinstantaneous displacement of slab 10 is identified by the low er case din Fig. 2. As shown in Fig. 3, the phase shift produced by slab'10increases'slowly at first as slab 10 is moved away from narrgw wall 16and into more intense electric field near the center of the waveguide;As dielectric slab 10 nears the center of the waveguide 18 the phaseshift increases more rapidly as shown by the upwardly concave shape ofthe curve 39 of Fig. 3.' The phase shift produced 'by the single slabreaches a maximum with the slab 10 located on the longitudinal airis ofthe waveguide 1 8. Further movement of slab 10 toward na frow wall 14causes a decrease in the phase shift, the decrease being rapid at firstand becoming more gradual as slab 10 approaches wall 14. Curve 39 issubstantiallysymmetrical about a line passing through the peak parallelto the phase shift axis. As pointed out above, the variation in phaseshift near the central portion of waveguide 18 is so rapid that, evenwith a micrometer adjustment as shown in Fig. 1, an accurate reading ofthe phase shift introduced by a single slab phase shifter cannot alwaysbe obtained. 2

Fig. 4 illustrates the phase shift produced by the novel 'phase shiftershown in Figs. 1 and 2. It will be noted one-half themaximurridisplacement shown in Fig. 3. This resultsifrom' the fact thatwhen two dielectric slabs are employed, the movement of the double slabphase shifting element through approximately one-half the width D of thewaveguide 18 brings one or the other of the dielectric slabs 10 and 12into contact with a narrow wall of the waveguide. The minimum phaseshift position of the embodiment shown in Figs. 1 and 2 is at the pointwhere di,electric slabs 10 and 12 are symmetrically positioned withrespect to the center of the waveguide 18. At this position'the totalphase shift introduced will be substantially equal to twice the phaseshift introduced by either of the two slabs taken separately. .As theslabs 10 and. 12 are moved from this minimum phase shift position byturning sleeve 34, one of the slabs, for example slab 10, approaches thecenter of the waveguide while the other slab, in this'case slab 12,moves toward the nearer narrow wall 16. Theincremental change in phaseshift produced by slab.10 is relatively large since, as explained above,the phase shift produced by a slab increases rapidly as theslabapproaches the center of the waveguide. The movement of the slab 12toward wall 14 causes a decrease in the phase shift produced by thisslab. However, the incremental decrease resulting from the movement ofslab 12 is less than the incrementalincrease produced by slab 10 sinceslab 12 is moving towards a relatively flatter portion of the curveshown in Fig. 3. Therefore movement 9f the phase shifting elements inthe manner outlined above will causea net increase in the total phaseshift produced, but this increase will be small and equal to the and thedecrease produced by slab 12. a

. The variation in phase shift with displacement for th phase shiftershownin Figs. 1 and 2 is illustrated by the solid line 40 in Fig. 4.Itwill be noted that this curve is relatively flat near the centerbutrises more sharply near the edges. It can be demonstrated, however,that, even near the ends of curve 40, the rise is less sharp than themaximum rate of rise shown in the curve of Fig. 3. To appreciate thereason for this, it is only necessary to recall that the incrementalphase shift produced by'the ifference between the net increase producedby slab 10 embodiment of Figs. 1 and 2 is always equal to the differencein phase shift produced by slabsw and 12 and the phase shift of one ofthese slabs is always decreasing as it approaches the narrow wall of thewaveguide. The flattened characteristic of curve of Fig. 4 permits veryaccurate adjustment of the phase shift since a relatively large changein position of slabs 10 and 12 is required to produce a small net changein phase shift.

Still further flattening of curve 40 may be produced at the expense of aslightly reduced range of adjustment by spacing slabs 10 and 12 by morethan one-half the broader dimension D of waveguide 18. By operating thedielectric slabs in regions displaced towards the extremities of thecurve shown in Fig. 3 the decrease in phase shift produced by one of theslabs will more nearly equal the increase of the phase shift produced byother of the slabs. However, as suggested above, the range of movementof the two slabs within the waveguide will be somewhat less than thatshown in Fig-s. 1 and 2.

Another means for still further flattening the curve 40 shown in Fig. 4is illustrated in Fig. 5. In the embodiment shown in Fig. 5 slabs 42 and44, which may be identical to slabs 10 and 12 shown in Figs. 1 and 2,are mounted on transverse rods 46 and 48 which pass through openings inthe narrow walls 50 and 52 of waveguide 54. Di-

electric slab 42 is securely fastened to rod 48 but is slida ble on rod46. Similarly, rod 46 is securely fastened to dielectric slab 44 but isslidable in a hole in dielectric slab 42. Therefore slabs 42 and 44 maybe moved independently of each other by appropriate movement of rods 44and 46 respectively. Additional supporting rods, longitudinallydisplaced from rods 44 and 46, may be provided to give additionalstability to slabs 42 and 44.

Two cams 56 and 58 mounted on a common shaft 60 engage the ends of rods46 and 48. The end of rod 46 is so formed that rod 46 engages cam 56along a line passing through the axis of shaft 60. A study of cams 56and 58 will show that each of these cams is shaped to cause one of thedielectric slabs to move from a position adjacent the narrow wall of thewaveguide to a position substantially coincident with the longitudinalaxis of the waveguide 18. The shapes are such that, regardless of theposition of shaft 60, whenever this shaft is rotated the dielectric slabnearer the center will move more slowly than the dielectric slab nearerthe narrow wall'of the waveguide. Preferably, the change in phaseshiftintroduced by the dielectric slab nearer the center of thewaveguide is greater than the incremental change in phase shift producedby the other dielectric slab so that a net increase in phase shift isproduced by moving one of the slabs towards the center of the waveguide.The total change in phase shift with rotation of shaft 60 will varydepending upon the particular shapes chosen for cams 56 and 58. Apossible curve of phase shift versus displacement of one of thedielectric slabs is shown by the broken line 62 in Fig. 4.

The mechanical arrangement shown in Fig. 5 is not necessarily the mostdesirable form for producing nonuniform movement of the dielectricslabs. Qther forms for producing a similar motion, for example lead.screws of uniform pitch driven by elliptical gears, may be preferredover the arrangement shown. Such mechanical expedients are well known inthe art and are sufiiciently identified by stating that the motionproduced should be such that the dielectric slab nearer the center movesmore slowly than the dielectric slab nearer the narrow wall of thewaveguide, and that the phase shift'produced by the slab nearerthecenter should be greater than the phase shift produced by the slabnearer the narrow wall.

It will be noted that the variation in total phase shift obtainable withthe embodiment of Fig. 1 is somewhat smaller than the total variation inphase shift obtainable with a single dielectric slab phase shifter. Thetotal range of phase shift produced with a double slab phase shifter canbe controlled to a certain degree by properly selecting both thethickness and length of the dielectric constant of the slab.Furthermore, the limited rangeof phase shift produced should not beconsidered a disadvantage since in many instances the total phase shiftto be introduced into a circuit remains relatively fixed with but asmall change in total phase shift being required to balance out smallirregularities produced by other circuits.

The adjustable double strip attenuator which forms a part of the presentinvention may have a supporting structure similar to that shown in Fig.l or Fig. 5. The two slabs or strips in the attenuator are formed ofdielectric carrier plates coated with carbon or metalized-glass platesof suitable length and cross-section. The ends of the resistive stripspreferably are tapered to prevent the reflection of energy therefrom.The curve of Fig. 3 illustrates the change in attenuation withdisplacement of the resistive strip in a conventional sliding vaneattenuator. The differential change in attenuation is obtained inexactly the same manner as the difierential change in phase shiftdiscussed above. Therefore Fig. 4 also illustrates the change ofattenuation with displacement of the two resistive strips in theattenuator embodiments of the present invention.

I am aware that other changes and modifications may be made in theembodiments disclosed herein without eparting from the spirit and scopeof the invention. For example, the two dielectric slabs or resistivestrips may have dissimilar cross-sections, different dielectric constants or resistivity or different lengths. The number of supportingrods may be varied or the entire supporting structure may be modifiedand the cross-section of the waveguide may be other than rectangular.Therefore, the embodiments disclosed herein illustrate only what is atpresent considered to be preferred forms of the invention, the fullscope of the invention being defined by the hereinafter appended claims.

What is claimed is:

l. A phase shifting device comprising a section of rectangular waveguidedimensioned to propagate the energy to be phase shifted in the dominantmode, first and second elongated dielectric slabs of substantiallyrectangular transverse cross-section, the cross-sectional area of eachof said slabs being small compared to the crosssectional area of thewaveguide, the broader faces of each of said dielectric slabs beingdisposed parallel to the narrower walls of said waveguide and said twodielectric slabs occupying substantially identical longitudinalpositions within said waveguide, a plurality of spaced supportings rodseach disposed substantially parallel to the longer transverse axis ofthe Waveguide, the narrower walls of said waveguide being apertured toreceive said supporting rods, said dielectric slabs being joined to saidsupporting rods for transverse movement within said waveguide and saiddielectric slabs being spaced apart by approximately one-half of thelonger inside dimension of said waveguide, and means associated withsaid supporting rods for micrometrically varying the position of saiddielectric slabs.

2. An adjustable circuit element comprising a section of dielectricfilled rectangular waveguide, first and second elongated slabs ofmaterial having electrical properties substantially different from theelectrical properties of the dielectric within the waveguide, said slabsbeing longitudinally positioned within the waveguide with the majorfaces thereof parallel to the electric field within the waveguide, meansmaintaining a fixed transverse spacing between said two slabs, saidspacing being not less thanhalf the longer transverse dimension of saidwaveguide, and means for-varying the transverse position of said: slabswithin said waveguide.

3. An adjustable attenuator comprising a section of rectangularwaveguide, first and second elongated slabs of resistive material, saidslabs being longtitudinally positioned within the waveguide with themajor faces there of parallel to the electric field within the waveguideguide.

means maintaining a transversespacing between said twb slabsisaidspacing being not less than half the longer transverse dimension of saidwaveguide, and means for varying the transverse position of said slabswithin said waveguide. i V

4. An adjustable phase shifter comprising a section of rectangularwaveguide, first and second elongated slabs of dielectric material, eachof said slabs being longitudinally positioned within the waveguide withthe major faces thereof parallel to the electric field within thewaveguide, means maintaining a fixed transverse spacing be tween saidtwo slabs, said spacing being not less than the half of the longitudinaltransverse dimension of said waveguide, and means for varying thetransverse position of said slabs within said'waveguide.

5. An adjustable circuit element comprising a section of dielectricfilled rectangular waveguide, first and second elongated slabs ofmaterial having electrical properties substantially difierent from theelectrical properties of the dielectric within the waveguide, said slabsbeing lon gitudinally positioned within the waveguide with the majorfaces thereof parallel to the electric field within the waveguide, meansfor varying the transverse position of said slabs within said waveguide,said positioning means being constructed and arranged to maintain thespacing between said two slabs approximately equal to one-half thelonger transverse dimension of said waveguide, the variation, if any, insaid spacing over the operating range of said circuit element being asmall fraction of the average spacing between said slabs.

6. The adjustable circuit element of claim wherein said slabs are formedof resistive material thereby to cause said circuit element to functionas an adjustable attenuator.

7. The circuit element of claim 5 wherein said slabs are formed of adielectric material whereby said adjustable element is caused tofunction as a variable phase shifter.

' 8. An adjustable circuit element comprising a section of rectangularwaveguide provided with a fiuid dielectric tberewithin, first and secondelongated slabs of material having electrical properties substantially.difierent from the electrical properties of the dielectric within saidwa eguide, said slabs being longitudinally positioned within thewaveguide with the major faces thereof at all times parallel to theelectric fieldwithin said waveguide, means for varying the position ofsaid first slab in the direction of the longer transverse axis of saidwaveguide and within the region bounded by one narrow wall and thelongitudinal axis of said waveguide, means for varying the position ofsaid second slab in the direction of the longer transverse axis of saidwaveguide and within the region bounded by the other narrow wall andsaid longitudinal axis of said waveguide, said two last-mentioned meansbeing so constructed and arranged that as either slab moves toward .thelongitudinal axis of said waveguide, the other slab moves away from saidlongitudinal axis, said two last-mentioned means being furtherconstructed and arranged so that the instantaneous rate of movement ofthe slab nearest said longitudinal axis is not greater than theinstantaneous rateof movement of the slab farthest from saidlongitudinal axis and the variation, if any, of the relative transversespacing between said two slabs is much less than one-half the longertransverse dimension of said wave- 7 "9. The adjustable circuit elementof claim 8 wherein said last-mentioned means is so constructed .andarranged that the slab nearer the longitudinal axis of said waveguidemoves more slowly than the other slab.

10.1811 adjustable circuit element comprising a sec, tion of dielectricfilled waveguide dimensioned to propagate the energy to be transmittedin the dominant mode, first and second elongated slabs of materialhaving electrical properties substantially difierent from the dielectricwithin the waveguide, each of said slabs being disposed with the longestaxis thereof .at all times parallel to the longitudinal axis of thewaveguide, means for supporting said slabs for movement toward and awayfrom the longitudinal axis of saidwaveguide, said means for moving saidslabs being so constructed and arranged that, as either slab movestoward the longitudinal axis of said waveguide, the other slab movesaway from said longitudinal axis, said means for moving said slabs beingfurther constructed and arranged so that the instantaneous rate ofmovement of the slab nearest said longitudinal axis is not greater thanthe instantaneous rate of movement of the slab farthest from saidlongitudinal axis and the variation, if any, of the relative transversespacing between said two slabs is much less than one half the longertransverse dimension of the waveguide.

11. The adjustable circuit element of claim 10 wherein said waveguidehas a rectangular cross-section.

12. The adjustable circuit element of claim 10 wherein said waveguidehas a rectangular cross-section and.

wherein said slabs are formed of a resistive material thereby to causesaid circuit element to function as a variable attenuator. a

13. The adjustable circuit element of claim 10 wherein said waveguidehas a rectangular cross-section and wherein said slabs are formed of adielectric material thereby to cause said circuit element to function asa variable phase shifter. 14. An adjustable circuit element comprising asection of rectangular waveguide provided with a fluide dielectrictherein, first and second elongated slabs of material having electricalproperties substantially different from the electrical properties of thedielectric within said waveguide, said slabs being positioned within thewaveguide with the major faces thereof at all times parallel to thenarrower walls of said waveguide, means for varying the positionof saidfirst slab toward and away. from the longitudinal axis in a directionparallel to the longer transverse axis of saidwaveguide and within theregion bounded by one narrow wall and the longitudinal. axis of saidwaveguide, means for varying the position of said second slab toward andaway from the longitudinal axis in a direction parallel to said longertransverse axis and within the region bounded by the other narrow walland said longitudinal axis of said waveguide, said last-mentioned twomeansbeing so constructed and arranged that, asfeitherslab moves towardthe longitudinal axis of the waveguide, the other slab 'moves away fromsaid longitudinal axis, said two lastmentioned means being furtherconstructed and arranged so that the instantaneous rate of movementofthe slab then causing the greatest change in electrical characteristicper unit change of slab position is not greater than the instanta' neousrate of movement of the other slab and the variation, if any, of therelative transverse spacing between said twoslabs is much less than thelonger transverse dimension of said waveguide.

References Cited in the file of this patent V UNITED sra'rns PATENTS2,491,662 Houghton Dec; 20, 1949 2,602,857 Hewitt July 8, 1952

